Electrochromic devices are often used as windows, shades, dividers, mirrors, or electronic displays, that change color or degree of opacity in respect to an applied electric field or current. Such an electrochromic device typically is a multi-layer assembly. Outer layers of the electrochromic device typically are electrodes that are optically clear [i.e. substantially transparent or translucent to light in wavelengths of the visual spectrum or at other desired wavelengths, albeit in some instances bearing a limited tint or color]. At least one electrochromic layer is sandwiched between the electrodes. This layer is able to change color or opacity in response to changes in the applied electric field or current to create visual effects. The electrochromic layer is often an organic polymer film or an inorganic thin film of an electrochromic material. When the voltage is applied across the outer conductors, ions in an electrolyte typically move to the electrochromic layer causing the electrochromic material to change color states. Reversing the voltage moves ions away from the electrochromic layer, restoring the device to its previous state.
An electrolyte is often used in an electrochromic device to act as a reservoir for the ions that activate the electrochromic layer and/or provide a medium for transporting ions between a separate ion reservoir material or counter-electrode and the electrochromic layer. A salt such as lithium perchlorate (LiClO4) or trifluorosulfonimide (LiN(CF3SO2)2) may be used to provide the ions to activate and deactivate the electrochromic layer. The salt is typically dissociated in a solvent in the electrolyte, freeing the ions for use in activating the electrochromic layer.
Gel electrolytes in electrochromic devices are often preferred because they are less likely to leak than liquids and more stable dimensionally. One gel electrolyte usable in a preferred electrochromic device includes a solid polymer matrix, especially of polymethylmethacrylate (PMMA).
It is desirable for the electrolyte to have high ionic conductivity, permitting the ions to move within the electrolyte, while having relatively low electric conductivity so that the electrochromic device does not short out. Prior art solvents for electrolytes for electrochromic devices include acetonitrile and/or ethylene carbonate. However, many of the solvents used for electrochromic devices have comparatively high vapor pressures and are comparatively volatile and thus can evaporate, and/or are unstable, have higher flammability, and/or have higher toxicity. Evaporation of a solvent in an electrolyte can change the electrolyte composition and degrade functionality of the electrochromic device.